1. Field of the Invention
The present invention relates to a valve timing adjusting apparatus that adjusts valve timing of at least one of an intake valve and an exhaust valve of an internal combustion engine that are opened and closed by a camshaft upon transmission of a torque from a crankshaft.
2. Description of Related Art
In one known valve timing adjusting apparatus, the valve timing is adjusted by changing a relative rotational phase between two rotatable bodies, which are rotated synchronously with the crankshaft and the camshaft, respectively. For example, DE4110195C2 discloses a valve timing adjusting apparatus, which changes a relative rotational phase between two rotatable bodies through use of a differential gear mechanism, which includes a planet gear as its main component. Specifically, in the apparatus of DE4110195C2, two inner gears are provided to the rotatable body synchronized with the crankshaft and the rotatable body synchronized with the camshaft, respectively, and axial extents of these two inner gears do not overlap with each other. These inner gears are meshed with two outer gears, respectively, of the planet gear. In this way, a large speed reducing ratio can be obtained with the compact design.
In the apparatus disclosed in DE4110195C2, the inner gear (hereinafter, referred to as a camshaft-side inner gear), which is provided to the rotatable body synchronized with the camshaft, directly receives the oscillating torque of the camshaft. Thus, a relatively large surface pressure is generated in a tooth contact part between the camshaft-side inner gear and the corresponding outer gear. In contrast, the inner gear (hereinafter, referred to as a crankshaft-side inner gear), which is provided to the rotatable body synchronized with the crankshaft, receives a reduced oscillating torque due to the action of the differential gear mechanism, which is formed by the crankshaft-side inner gear in cooperation with the camshaft-side inner gear and the planet gear, so that a relatively small surface pressure is generated in a tooth contact part between the crankshaft-side inner gear and the corresponding outer gear. Furthermore, the torque of the crankshaft, which uniformly rotates the camshaft, is normally smaller than the oscillating torque of the camshaft, so that the surface pressure generated in the contact part between the crankshaft-side inner gear and the corresponding outer gear should be reduced.
However, in the apparatus disclosed in DE4110195C2, the length of the tooth contact between the camshaft-side inner gear and the corresponding outer gear measured in the direction of the tooth trace thereof is smaller than the length of the tooth contact between the crankshaft-side inner gear and the corresponding outer gear measured in the direction of the tooth trace thereof. In this way, the tooth contact surface area between the camshaft-side inner gear and the corresponding outer gear is made smaller than the contact surface area between the crankshaft-side inner gear and the corresponding outer gear. Therefore, the camshaft-side inner gear and the corresponding outer gear need to withstand the relatively large surface pressure generated in the tooth contact part between the camshaft-side inner gear and the corresponding outer gear. This can be accomplished by increasing the rigidity of the camshaft-side inner gear and of the corresponding outer gear by, for example, use of a highly rigid material or a hardening treatment. However, this will result in an increase in the manufacturing cost.